The present invention generally relates to spray applicators for forming and projecting a CO2 Composite Spray (a trademark of CleanLogix LLC). More specifically, the present invention relates to a passive electrostatic spray nozzle and spray applicator assembly employing air, solid carbon dioxide, and additive particles such as organic solvents, coatings, paints, nanoparticles, microabrasives, and lubricants.
Use of CO2 composite sprays for cleaning, cooling and/or lubrication is widely known in the art. For example, CO2 composite sprays are typically employed during hard machining processes requiring cleaning, selective thermal control, and/or lubrication during turning, precision abrasive grinding, or dicing operations. In these applications, CO2 composite sprays are employed to extend cutting tool or abrasive wheel life, and to improve productivity, dimensional tolerance, and surface finish.
There exist in the art several examples of CO2 spray applicators which are employed to direct a CO2 spray onto substrates, work pieces, and the like, in manufacturing or industrial processes. Such examples include U.S. Pat. Nos. 4,389,820, 4,806,171 and 5,725,154. Each of the aforementioned, however, have shortcomings in the application of sprays for cleaning, cooling and lubricating purposes, more especially the formation and application of CO2 composite sprays beneficial for cooling and lubricating purposes.
For example, efficient and effective application of CO2 composite sprays to machined substrates presents several challenges. When sufficiently high spray velocities are employed to provide enough energy to reach cutting zone surfaces, the majority of the spray tends to deflect from or stream around the cutting zone surfaces rather than impinge upon them. When low velocity sprays are employed, critical surfaces with recesses or complex surfaces cannot be penetrated effectively. For example, during application of CO2-based cooling-lubricating sprays it is observed that oil additive agglomerates into very large precipitations during transition from spray nozzles to surfaces. This phenomenon interferes with the even distribution of both CO2 coolant particles and oil-based lubricant on machined surfaces and causes a large portion of the atomized spray to miss the substrate entirely if positioned at a location too far away from the substrate being machined, wasting a portion of the applied spray. This phenomenon occurs because the lubricating additive, such as an oil, and a cooling component, solid carbon dioxide particles, have certain physicochemical properties which are in complete opposition—namely high melt point and extremely low temperature, respectively. The temperature of the CO2 particles (i.e., coolant) cause a flowing lubricant additive to solidify or gel prematurely before a uniform particle size and spray distribution can be established within the spray. This phenomenon inhibits uniform and homogenous dispersions. This is particularly the case when the mixing between the CO2 solid particles and additive particles occurs within the nozzle or near the nozzle tip, resulting in inconsistent spray patterns and chemistry, and the nozzle becoming clogged with frozen and agglomerated oil and additives.
The prior art contains several examples of CO2 spray application techniques for incorporating beneficial additives into a CO2 composite spray. Examples include the addition of organic solvent additives to enhance spray cleaning performance, lubricant additives to enhance machining performance, and plasma additives to enhance surface modification for adhesive bonding. Examples of prior art in this regard include U.S. Pat. Nos. 5,409,418, 7,451,941, 7,389,941 and 9,352,355. In each of the aforementioned examples, an additive fluid comprising ions, solvent, oil, or a plasma, respectively, is added directly into a centrally disposed CO2 particle spray using an injection means that is integrated with the CO2 spray nozzle device, and in some cases include a means for actively charging the additive particles using high voltage and an electrode to enhance additive particle attraction, mixing and atomization. However, as already noted this type of injection scheme introduces constraints for spray additives which are inherently incompatible with the physicochemistry of the CO2 spray at or near the spray forming nozzle. For example, high spray pressure and velocity, very low temperature, and passive electrostatic charging within the CO2 particle nozzle body and exit introduce flow and mixing constraints for high melt point oils. High molecular weight natural oils such as soybean and canola oil provide the most superior lubrication qualities for machining applications but will gel or solidify at temperatures much higher than those present within or near the CO2 particle nozzle exit. Exacerbating this problem is electrostatic fields and charges present during the formation and ejection of CO2 particles within and from the nozzle. Spray charging using a high voltage electrode or passively charging (tribocharging) the additive and/or CO2 particles, respectively, electrostatically charges and coalesces the subcooled high melting point oil films into large and sticky gels or masses near or within the nozzle tip which inhibits flow and injection into the CO2 particle stream. Moreover, these larger additive particle masses once injected into the cold CO2 particle stream and projected at a target surface inhibit gap penetration during to very low surface area, for example within a cutting zone comprising cutting tool, workpiece and chip crevice. The result is a spray with compositional variance over time—large particle masses with low surface area or a complete lack of lubricating particles. Moreover, the additive injection apparatus and methods of the prior art require an individual additive injection scheme for each CO2 spray nozzle necessitating more complicated multi-spray configuration schemes in applications requiring larger aerial and radials spray densities for increased application productivity or utility.